1. Field of the Invention
The present invention relates to the field of torque measurement; and more particularly, to means for determining the amount of twist that occurs in a shaft under torsion.
2. Description of the Prior Art
When torque is applied to a shaft, two principal lines of stress are induced along helical lines which are orthogonal to each other on the surface of the shaft. Various different methods of torque measurement have been available, but no method is totally satisfactory. Two common methods of measuring torque, strain gage and optical, are well described in the literature. See "Sensor and Analyzer Handbook," by Harry Norton, Prentice Hall, 1982, pp 131-142.
Torque is most accurately measured by bonding strain gages in a cross arrangement along the helical lines of compression and tension. The strain gages are electrically configured in a balance-bridge and coupled to measuring electronics via slip rings or noncontacting rotary transformers. Generally, these cross arrangements are difficult to implement and usually require custom installation. In another variation, disclosed by Gurenko and Krutkis in Soviet patent 2,493,268, a light source is used to couple the gage signal to a stationary photodiode.
In optical torque transducers, light beams, code patterns and light sensors are used to convert the differential angular displacement between two positions on a shaft, due to applied torque, into an output signal. Specifically, identical patterns made of light reflecting strips are arranged circumferentially around the shaft at two different locations. The patterns are illuminated by laser diodes and the reflected light is sensed by photocell. The output of each photocell is a pulse train and the phase difference between them is a measure of the torque. U.S. Pat. No. 4,767,925 to Kawamoto discloses a similar device in which a pair of light emitting and receiving elements produce an output depending on the amount of light transmitted due to the relative rotation of two slotted disks. U.S. Pat. No. 4,637,264 to Takahashi et al. discloses the use of two plates, each having an optical grid, which generate a Moire pattern that depends on the relative angle between the two plates. A connecting member is attached to the shaft solely at a point away from one of the slotted disks. The device disclosed by Takahashi et al. has a serious drawback in that the disk tends to oscillate since the connecting member is only supported at one end. These oscillations put the two disks out of parallel and introduce an error in the torque angle. Further the device of Takahashi et al. requires a hermetically sealed unit as well as an oil seal, since it is sensitive to dirt and dew. U.S. Pat. No. 4,433,585 to Levine discloses passing a beam of light through two diffraction gratings placed at different locations along a shaft and sensing the phase of the two resulting beams. U.S. Pat. No. 3,938,890 to Flavel discloses the use of two light transmission controlling disks, one of which is comprised of two areas. Each of the two areas of the plural area disk has a different direction of optical polarization, so that in combination, the controlling disks provide three areas in all. This configuration of Flavel makes possible a linear output for unspecified small angles of torque. The polarizing disks required by Flavel are expensive and the difficulty of alignment further increases the cost. Due to the complexity of the polarizing disks, very complicated detection means are required to increase sensitivity by eliminating aberrations in the polarizers. U.S. Pat. No. 4,650,996 to Maehara et al. discloses an angle transducer employing two polarizing disks, two light sources, and two light detectors. Maehara et al. further discloses the use of a mirror in order to have all sources and detectors on a common side of the disk. Even in its minimum configuration, Maehara et al. requires at least two light sources, and two light detectors. Since measurement of torque involves the determination of angle at two different locations along a shaft, two such devices as Maehara et al. describes would be required two measure torque. The optical torque sensors of the prior art are expensive, and not robust devices, as they require complicated detection schemes and exact alignment. Further they are not provided with a means for installation in the field.
The literature discloses a capacitive torque sensor consisting of two encoders either mounted perpendicular to the shaft at each end, or mounted circumferentially at two closely placed points along the shaft. See "Interest in Misfire Detection Technology Grows," Automotive Electronics Journal, Nov. 6, 1989, pg 12. Each encoder has two parts: a stator that consists of up to 256 radial fingers that are alternately charged; and a rotor that is mounted on the shaft. As the shaft turns, the rotor's potential switches between positive and negative at a frequency proportional to speed. A disk, in the center of the stator, electrically isolated from the charged fingers collects the signal. Like the optical torque sensor, the twist of the shaft is determined by measuring the phase difference between the two encoders. Also like the optical sensor this device requires exact alignment.
The magnetic properties of most ferromagnetic materials change with the application of stress to such an extent that stress may be ranked with field strength and temperature as one of the primary factors affecting magnetic change. Magnetostriction is a measure of the stress sensitivity of a material's magnetic properties. Magnetic based torque sensors take advantage of the magnetostrictive properties of ferromagnetic metals, such as carbon steel. See "Noncontact Magnetic Torque Transducer", Sensors, 11/90, pp. 37-40. These sensors make a contactless measurement of changes of magnetic permeability in shaft materials, which are caused by torsional stress. In place of strain gages, magnetic flux is directed into the shaft and along the helical lines of compression and tension. A positive magnetostriction shaft experiencing torsion will exhibit increased permeability along the line of tension and decreased permeability along the line of compression. At low stress levels the permeability is nearly linear with stress, but varies dramatically at high stress. Another drawback of a magnetostrictive torque sensor is in the need for calibrating it individually with each shaft. This requirement is obvious because the torque measurement is made by means of the magnetostrictive properties of the shaft material and cannot be predetermined in the manufacture of the sensor by itself. The variability in magnetostrictive properties is usually correlated with the variability of the mechanical hardness of the material. Hardness variability of shaft materials typically ranges from +10 percent to +40 percent. The shaft-to-shaft variability problem has been addressed by adding either a sleeve or coating of a well defined and magnetically soft material, such as nickel, permalloy, or ferromagnetic amorphous alloys. See Sasada, Hiroike and Harada, Intermag '84, BE-03, IEEE Transactions on Magnetics, MAG 20, V5, pg 951. While this approach shows promise, installation can not be made in situ, and all magnetic materials, even the softest, can retain some magnetism leading to nonlinearities and drift. In addition, magnetic torque sensors are susceptible to electromagnetic interference.